U.S. Pat. No. 3,984,710 which corresponds to DE-OS No. 25 13 007 describes a starting aid for a single-phase synchronous motor in which pressure rollers are urged against a cam from opposite directions and are arranged on slides so as to be movable towards and away from one another. This mechanism constitutes an additional starting aid and has no function in the actual drive system. By means of the spring-loaded pressure rollers the cam rotates the rotor of a single-phase synchronous motor after de-energization of the stator in such a way that starting is always quaranteed, even in the case of a high friction or, in other words, in such a way that the main direction of magnetization of the rotor is rotated relative to the main stator-field direction in such a way that the two directions make an angle with one another. This is because the motor cannot be started if the two field directions coincide.
U.S. Pat. No. 4,400,875 which corresponds to EP-OS No. 45 107 describes a vibration-type dry-shaver driven by means of a rotary motor. Two cam discs are arranged on the rotor shaft of this motor perpendicularly to one another and these cam discs are each followed with a roller to drive the cutter of the dry-shaver via an articulated lever system. This lever system is vulnerable because its parts are articulated to one another by means of integral hinges. Moreover, two cams arranged above each other on the motor shaft lead to a larger height, which is in conflict with the requirement to minimize the height of the apparatus. Another construction employing a rotary motor and a crank drive produces annoying noises.
Further, said U.S. Pat. No. 4,400,875 also describes a cam mechanism in which the cam has the shape of an equilateral triangle having three identical convexly curved sides, the vibratory motion being produced by the circumferential faces of the cam. The motion is transmitted by means of a U-shaped yoke having parallel limbs which extend on opposite sides of the cam. The cam profile is such that the distance between two arbitrary parallel lines tangent to the profile is constant. Such a device is known as a constant-diameter mechanism. Thus, as the cam rotates, the contact between the cam and yoke is constantly maintained within the parallel edges of the U-shaped yoke. It has been found that such a constant-diameter mechanism presents substantial problems with respect to tolerances, jamming and noise. The same applies to a double contact mechanism as described in U.S. Pat. No. 2,741,132.
Further it is known from French Patent Specification No. 872,634 in a machine tool comprising a cam-and-follower system to urge the cam-follower arm against the cam by means of a spring. The function of this spring is to maintain a steady contact between the arm and the cam under dynamic conditions. A similar construction of vibratory apparatus driven by synchronous motors is known from German Patent Application P No. 34 04 297.0 (PHD 84-022). The spring used in this construction serves to maintain the contact between the cam and a roller also under dynamic conditions and, in addition, to rotate the rotor into such a rest position that the main direction of magnetization of the rotor does not coincide with the main statorfield direction. For vibration apparatus this construction has the disadvantage that the large spring force necessary to maintain the contact between the roller and cam gives rise to substantial frictional losses in the vibratory system. These frictional losses consume a part of the motor power.
Further, it is also found that such vibratory systems produce annoying noises. A considerable part of such noises is produced by mechanical tolerances in the transmission path. These tolerances, which are partly inevitable, lead to uncontrolled cutter movements attended with noise.
It is known to combat noise by the use of constructions without backlash. This is possible, for example, if the fork mounted on the cutter is clamped around the pivotal-arm lever. The pressure spring employed in the construction in accordance with French Patent Specification No. 872,634 also serves to eliminate backlash.
Finally, German Patent Application P No. 34 04 27 PHD84024 describes a device comprising two rollers which are arranged on a pivotal arm and which cooperate with a mirror-symmetrical cam driven by a synchronous motor.
Such a construction does not require a spring in order to maintain the contact between the rollers and the cam. Losses in the vibratory system are then reduced accordingly. However, this does not solve the starting and noise problems.